Steel for use at elevated temperature



United States Patent George V. Smith, Ithaca, N. Y., assignor to UnitedStates Steel Corporation, a corporation of New Jersey No Drawing.Application July 2, 1957 Serial No. 669,431

3 Claims. (Cl. 75-126) This invention relates to heat-treated chromiumsteels tempered at a temperature between 1100 and 1150 F.

With respect to the composition limits, carbon in excess of .25% has aharmful effect on the strength level. The minimum chromium andmolybdenum contents are determined by the fact that lower amounts ofthese will not give the desired strength level nor, in the case ofchromium will the necessary corrosion resistance be obtained. The upperlimit of these elements is determined by the amount of delta ferritethat can be tolerated. If these amounts are exceeded, delta ferrite inexcess of 30% will be present at the austenitizing temperature andtherefore the steel will not be commercially forgeable.

A relatively few of the typical compositions tested are given in TableI.

having improved stress-rupture strength at elevated temperatures andmore particularly to heat-treated chromium steels of the 8% grade havingimproved creep to rupture stress.

Due largely to the development of high speed aircraft and missiles,there has been an urgent necessity for material possessing highstress-rupture strength at temperatures of the order of 1100" F. andwhich at the same time possess adequate corrosion resistance. Heretoforethere has been some use made of the age-hardening austenitic-chromiumsteels containing upwards of 16% chromium with about 7% nickel. Whilepossessing certain desirable qualities such as good corrosionresistance, such steels are quite high in price partly because of thelarge amount of expensive chromium and nickel alloying ingredients andpartly because of difiiculties in producing and processing them.

It is accordingly an object of the present invention to provide aheat-treated steel having the desired high temperature stress-rupturestrength with only sufficient chromium to provide adequate corrosionresistance.

It is a further object to provide heat-treated steel of the 8% chromiumtype having a l00-hour stress-rupture strength in excess of 50,000 p. s.i. at 1100 F.

I have discovered that such stress-rupture resistance can be obtained inthe 8% type chromium steel containing chromium, carbon, molybdenum,silicon and manganese within the following ranges, when heat treated inthe specific manner hereinafter described. More particularly the steelshould contain a maximum of carbon, 7.50 to 8.25% chromium, 2.70 to3.40% molybdenum, 3.00% maximum managnese and .75% maximum silicon, withthe balance iron and other elements in amounts which do not adverselyaffect the properties. Thus for instance the steel may contain up to .5%titanium, up to .4% vanadium and up to .l% boron without these elementshaving any marked effect on the creeprupture properties upon testing at1100 F. after air cooling from 2200 F. and tempering 2 hours at 1100 F.To develop the desired properties, such steels should be heated to anaustenitizing temperature between 1900 and 2200 F., transformed to amartensitic structure and Table 1 Steel 0 Mn S1 Cr Tl Mo V Al B N TableII Hr. Rupture Strength at 1.100 F., 1,000 p. s.1.

Steel Austenitizing Temperature, F.

The effect of varying heat treatment on the 100-hour creep-strength at1100 F. is shown in the following Table III:

The effect of varying heat treatment on the ZOO-hour creep-strength at1100" F. is shown in the following Table IV:

Table IV 200 Hr. Rup- Steel Heat Treatment ture Strength at 1,100 F.,1,000 p. 5.1.

1,900 A. O.-1,100 Temper. 52.0 2,000 A. C.l,100 Temper- 54.0 3 Z-70...2,100 A. (Jr-1,100 Temper- 53.4 2,200 A. C.1,100 Temper. 52.1 2,000 A.O.-1,150 Temper 50. 3

This indicates that the optimum heat treatment involves austenitizing atabout 2000 E, transforming to a martensitic structure and tempering atabout 1100 F.

This application is a continuation-in-part of my copending applicationSerial No. 583,619, filed May 9, 1956, now abandoned.

While I have shown and described several specific embodiments of myinvention, it will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised within the scope of my invention, as defined in theappended claims. a

I claim:

1. A forgeable ferritic chromium steel of the 8% type characterized by ahigh 100-hour stress-rupture str n th at 1100 F. in the quenched andtempered condition, said steel containing ;25% maximum carbon, 7.50 to8.25% chromium, 2.70 to 3.40% molybdenum, 3.00% maximum manganese, .75%maximum silicon with the balance iron and other elements in amountswhich do not adversely afiect the properties.

2. A forgeable hardened and tempered ferritic steel of the 8% chromiumtype characterized by a IOU-hour stress-rupture strength in excess of50,000 p. s. i. at 1100 F., said steel containing .25 maximum carbon,7.50 to 8.25% chromium, 2.70 to 3.40% molybdenum,

"4 3.00% maximum manganese, .75 maximum silicon with the balance ironand other elements in amounts which do not adversely affect theproperties, said steel having been austenitized at a temperature fromabout 1900 to 2200 F., transformed to a martensitic structure andtempered at a temperature between 1100 and 1150 F.

3. A forgeable hardened and tempered ferritic steel of the 8% chromiumtype characterized by a l00-hour stress-rupture strength in excess of50,000 p. s. i. at 1100" F., said steel containing .25 maximum carbon,7.50 to 8.25% chromium, 2.70 to 3.40% molybedenum, 3.00% maximummanganese, .75 maximum silicon with the balance iron and other elementsin amounts which do not adversely affect the properties, said steelhaving been air cooled from a temperature of about 2000 F. and temperedat a temperature of about 1100" F.

References Cited in the file .of this patent UNITED STATES PATENTS1,876,091 Strauss Sept. 6, 1932 FOREIGN PATENTS Molybdenum-Steels,Irons, Alloys, 1948, page 158. Published by the Climax Molybdenum Co.,New York, N. Y.

1. A FORGETABLE FERRITIC CHROMIMUM STEEL OF THE 8% TYPE CHARACTERIZED BYA HIGH 100-HOUR STRESS-REPTURE STRENGTH AT 1100*F. IN THE QUENCHED ANDTEMPERED CONDITIONS, SAID STEEL CONTAINING .25% MAXIMUM CARBON, 7.50 TO8.25% CHROMIUM, 2.70 TO 3.40% MOLYBDEMUM, 3.00% MAXIMUM MANGANSES, .75%MAXIMUM SILICON WOTH THE BALANCE IRON AND OTHER ELEMENTS IN AMOUNTSWHICH DO NOT ADVERSELY AFFECT THE PROPERTIES.